Modulator control



May 20 1924.

'R. V. L. HARTLEY MODULATOR CONTROL Filed Se t. 1919' p 27d]? cur/e07 Patented May 20, 1924.

'UNITED STATES PATENT OFFICE,

RALPH V. L. HARTLEY, OF EAST ORANGE, NEW J ERSEY, ASSIGNOR TO WESTERN ELEC- TRIC COMPANY, INCORPORATED, OF NEW YORK,'N. Y., A CORPORATION OF NEW YORK.

MODULATOR CONTRO L.

Application filed September 29, 1919. Serial No. 327,329.

To all whom it may concern:

Be it known that I, RALPH V. L. HARTLEY, a citizen of the United States, residing at East Orange, in the county of Essex, State of New Jersey, have invented certain new and useful Improvements in Modulator Controls, of which the following is. a full, clear, concise, and exact description.

The present invention relates to the operation of relays in which a plurality of output-frequency components are produced for a given input wave.

A feature of the invention relates to the proportioning of the elements concerned in the operation of known relays or relay systems to emphasize certain of the components and suppress other components.

It'has been shown, for instance, in British Patent No. 102,503, that modulators can'be made to suppress completely the components of unmodified frequency, i. e., the carrier and the modulating frequencies, if the thermi onic devices which constitute the modulator are exactly equivalent; Thermionic devices have been combined for other purposes, such as amplifiers of the push-pull type in which the combination frequencies are suppressed to a large extent, or as harmonic generators. It is generally assumed in all such cases that either the thermionic devices are equivalent or that the suppression of the undesired component is only partial. The

equivalency of the thermionic devices is a matter of degree and there may be times when it isessential that the suppression of certain components ,be as complete as possible; for instance, in secret systems of communication in which a Small leakage of a carrier frequency, for example, might serve as a key to the carrier that is being used.

According to the present invention, the various quantities which influence the output current components are proportioned in predetermined manner to control the nature of the output currents. vFor the purpose of showing how and why these quantities are to be proportioned, a mathematical analysis of the action of the typical thermionic threeelectrode relay will be given and reference will be made to the accompanying drawings in which Fig. 1 gives curves characteristic of the type of relay discussed, and Figs. 2

and 3 give schematic circuit arrangements for the relays.

It has been shown that the output current I from a three-electrode thermionic relay may be quite accurately expressed within certain limits by the equation.

where a is a constant depending upon the structure of the relay, u, is the maximum voltage amplification obtainable from the device, E is the voltage existing between the anode and the cathode, E is the voltage existing between the cathode andthe grid electrode, and E is a small correction term to take care of a number of factors, such as contact differences of-potential in the relay and the power developed in the filament.

For further reference the article by H. J. Van der Bijl in the Physical Review for September, 1918, page 171 et seq, may be consulted. I

If the external impedance of the relayis so small that the voltage drop, due to the external current, is small in comparison with the impressed voltages, equation (1).

may be expressed in the form 1=a( E,+E. +E+e.)

where'e now represents the impressed vari able grid potential for controlling the output current I.- This relation holds only for values within certain limits. If (E+e) o,

I is zero for all such "alues, and if (E-i-e) is positive and above a certain fairly large {epic or if very small, the relation does not If the relay is to be used as a modulator, the input voltage will consist of a carrier wave e,, to be modulated and a modulating wave 6. or e=e +e where ra -=1? cos pt and e =Q cos qt, P and Q, being'respectively the maximum amplitudes of the carrier and modulating-waves. Substituting in (3) and expanding The first term aE represents a fixed value and need not be here considered, the second and third terms represent the output of the unmodified carrier and modulating frequencies, respectively, the fourth term represents the double frequencies of the two input waves, respectively, and the last term represents the modulated output current, i. e. a wave of carrier frequency the amplitude of which varies in accordance with the wave e That the fourth term represents the double-frequency components may be seen by considering that e 1): (Rcos e =1 00 2 MP .9 2 1 1) in which cos 2 pt is seen to indicate a frequency double that of the wave P cos pt. That the last term represents the modulated component may be shown by substituting for 2 e e their equivalents so that the term becomes 2 PQ cos pt cos qt, which by trigonometric transformation becomes Q COS (2 +q) Q cos (2 single side frequencies, two side bands of frequencies, respectively,

and

as is well known, having amplitudes vary-.

ing in accordance with the amplitude of the modulating, or voice, wave.

Referring to equation (4),'it is seen that the terms representing the unmodified frequencies are dependent onE, while both the double-frequency and modulated compo nents are independent of E. Diminishing E, at least within the limits in which the relation expressed by equation (3) holds, docs not therefore affect the output of the combination frequencies but directly decreases the output of the single frequencies. By controlling E, therefore, the relay may be made to act selectively to a certain extent in determining the amount of unmodified carrier frequency or modulating frequency that is to appear in the output circuit.

In Fig. 1 the three curves A, B and C may represent part of the family of characteristic curves for a given thermionic relay, in which the ordinates represent the current between cathode and anode and the. abscissa represent the voltage of the grid with respect to the cathode. The curves A, B and C are obtained by using successively larger plate voltages, and it is obvious that a new curve will be obtained for every different plate voltage that is used. Considering that the portions ofcurves B and C, lying between the two limits a and 5, conform to equation (4), if the grid potential is varied within these limits by the input waves e and e. the output of the combination frequencies will be the same whether the plate voltage is that corresponding to,

curve B or to curve C, but in case of curve C the output of the unmodified frequencies will be greater. Obviously, then, if either the modulation or the double-frequency output alone is desired for any given grid voltage, the smallest plate voltage that will give the desired output of the modulation;

or double-frequency component should be used. This will materially cut down the production of the unmodified components.

A similar effect will be produced by variation of the normal grid potential, leavlng I the plate voltage constant. Such variation willnot, however, affect the output of the combination frequencies as has often been supposed heretofore, solong at least as the assumed equation holds. Either decreasing the plate voltage or making the grid more negative means a decrease in E. v

A practical application of the principles that have been set forth'may be realiztd'in a single modulator where a maximum production of modulated output is desired and a normal amount --of carrier component. Frequently considerable inefficiency. results from the fact that the carrier wave is only partially modulated and needless amounts of the carrier wave are transmitted. In Fig. 2, 1f the modulator 1 is fed from the source 2 of carrier frequency and with output of the unmodified carrier component ways of applying the same waves 6 and may be cut down. Also it isshown above that in the output the unmodified components are respectively proportional to P and Q, the double-frequencles are proportional to J? and Q respectively, and the modulated component is proportional to PQ. If the smallest value of i. e., the lowest frequency of the modulated component represented in the lower side band, is greater than the greatest doublefrequency of the modulating wave, a filter 5 may be used to pass only the carrier wave and the modulated components. By making P small and Q. correspondingly larger, the modulated components will be undiminished and the carrier component may be cut down to a desired value. As stated, the output of e e and (2,, can, underthe conditions assumed, be eliminated by the filter 5.

The principles above set forth find greater application where two or .more thermionic relays are combined in the form of balanced modulators,'amplifiers, etc. There are four q to therespective relays I, II, IIIand IV as follows:'

That is, by suitable input connections between the wave sources and the respectlve I relays, one or the other or both impressed first relay is then, neglecting the constant term aE,

+ 2a e e 5 By combining the outputs of any two relays so as to add or subtract the output products, two of the output products can be retained and the other two eliminated if the E value and the a value is the same for both of the relays used. Thus assuming a,=a,=a,

it i 4 (aEe Q p q) eliminates all except the modulated component and the unmodified component of the modulating Wave. retain only the modulation component and the unmodified carrier com onent. i,+,

would retain both combinatlon' frequency suming that t e relays 10 and 11 are equivalent. Interchanging the source 6 with the circuit 9 would give a formof modulator corresponding to the connection i,i in which the modulating-frequency component would be suppressed.

In practice it is diflicult, if not impossible, to obtain two thermionic relays with precisely the same constanta, but it is possible to make E the same for both relays. Taking into account this difference in a, i +i,, for' example, becomes Similarly, 71 -11 would If it is desired, for instance; to suppress completely the modulatin frequency and not to have the unmodi ed carrier component unduly large, the amplitude of the carrier Pv should be increased and Q should be decreasedin the same proportion. The modulated output which is proportional to PQ. would remain unchanged. By making E as small as possible and choosing a, as nearly equal as possible to a, the unmodified component of e may be made practically zero and the unmodified component of 0,, may be made small. Similarly, by increasing Q, relative to P and by making E small the unmodulated carrier frequency component could be made practically zero.

A modulator to which equation (6). applies quencies by the same relay combination i 11,: 01 [new 2Ee Z 1 In this case E should be large to give a large output of the unmodified frequencies and the completeness with which the combination frequencies willbe eliminated will depend upon how nearly equivalent :2 1s to 1 Increasing E within the limits imposed .by equation (3) will selectively increase the amplification of the unmodified frequencies but will have no efiect 'upon the undesired components.

By using all four relays with their input and output circuits properly related, any one of the output components can be completely isolated provided the ozs of all the tubes have negligible difi'erence. Thus to' isolate the modulated component the tubes would be connected in accordance with the relation '5 -'i, i '5 ='(8ae e which assumes that 0: is the same for all four tubes. Similarly, the component of pure double frequencies could be made to occur separately. This use-of four relays to secure a single output frequenc component from two impressed waves is escribed and claimed in my copending application, Serial No. 325,731, filed September 23, 1919.

It is to be understood that the invention isnot to be limited to'the particular circuits or to'the specific type of relay, or to the exact type of characteristic curves that have been shown and described, but only by the scope of the appended claims.

What is claimed is: l

1. The method of operating a three-electrode relay for transmltting a complex wave to control selectively the transmitted current components, which comprises proportioning the fixed voltages to control the output of unmodified-frequency components relative to the combination-frequency components, and proportioning the variable applied voltages to control the unmodified-frequency components relative to each other.

further decrease oi the plate V0 2. The method of operating a three-electrode modulator to suppress unmodified current components while producing a substantially' constant amount of modulated components, which comprises applying a normal negative voltage to the grid and varying either the grid voltage or the plate voltage in the negative direction until the total output current is diminished to the 1point where tage begins meaeos without some of the frequencies-modulating others, the pushull type of amplifier is ordinarily used. nection is (ez 2 +2 ape to cut down the production of modulated components.

3. The method of operating a three-electrode modulator to suppress unmodified current components while producing a substantially constant 'amount of modulated components, which comprises operating the modulator with a normal plate voltage and a grid potential adjusted to substantially such a value that-a change to a more negative value will cause a decrease in the output of the modulated components.

4. The method of controlling modulating discharge devices to transmit selectively the difierent frequency-components which comprises connecting said relays in balanced relation, and diminishing the normal effective constant voltages to the point where further decrease begins to cause distortion of the combination frequencies.

5. The method of suppressing the output of unmodified frequency-components in ,a balanced modulator consisting of discharge devices, which comprises connecting said devices so as approxlmately to eliminate the transmission of said components, and operating said modula.tor near the lower limit of the discharge current value for which the output of modified-frequency components remains substantially constant.

6. In a modulator for modulating a carrier wave by a modulating wave, said modulator comprising discharge devices, the method of suppressing one of said waves while keeping the component of modulated current constant, which comprises connecting said relays so as approximately to eliminate the transmission of the undesired wave, operating said modulator near the lower. limit of the discharge current value at which the modulated current component remains substantially constant, and making the amplitude of the undesired wave small in comparison with that of the other wave, still keeping their product practically constant.

Y. lln a balanced modulator for modulating a carrier wave by a modulating wave, the method of suppressing one of said waves while keeping the component of modulated current substantially constant, which comprises decreasing the amplitude of the wave to besuppressed, increasing the amplitude of the other wave in the same proportion, and operating the modulator near the lower limit or" the discharge current range within he equation for this concos qt, substantia which the modulated current component remains practically constant.

8.. The method of operating thermionic relays to control suppression of .certain frequency-components, which comprises connecting said relays so as to'neutralize said components except for inherent difierences in the relays, and proportioning the applied voltages which affect selectively the production of said components so as to cause approximately minimum ratio of said componentsto desired components.

9. The method of operating thermionic relays to control the suppression of certain current components, which comprises proportioning the fixed applied voltages to control selectively, the production of the unmodified frequency components, and proportioning the amplitudes of the applied waves to determine the minimum ratio of certain of the unmodified-frequency and multiple-frequency components to the combinatlon-frequency components.

10. The method of operating thermionic relays for which the equivalent 'fixedvoltages may be represented by E and the impressed waves by P cos pt and Q cos qt, respectively, which comprises proportioning E to give the desired production of unmodified-frequency components, and in proportioning P and Q, independentlyto control the production of one multiple-frequency component relative to another and dependently to control the production-of the combination-frequenc components PQ, cos pt lly as described.

11. Themethod of controlling two modulator relays 'to secure a substantially pure output wave of combination frequencies which comprises opposing-the transmission characteristics of one relay for unmodified frequencies to that of the other relay, and in applying the same fixed voltages to both relays of such value that the net resultant transmission of the unmodified frequency components is made substantially a minimum while the output of the combination frequency components 'is substantially undiminished.

12. In balanced relay systems for suppressing transmission of undesired frequency components in which the transmission property of one relay for undesired components is made to oppose that.of another re ay, method of suppressing the undesired frequencies which are not suppressed by virtue of the difl'erences in the respective relay transmission properties, comprising controlling the applied voltages to affect the transmission of the undesired components substantially independently of the other components.

the

1-3. In frequency modifying relay systems, the method of controlling selectively the output of unmodified frequency components comprising controlling the magnitude of the applied voltages which affect the output of the unmodified components practicallyindependently of the other components substantially as described.

I 14. In the use'of frequency-modifying relays having a direct current voltageoperating range within which the output .of complex frequenc components is substantially the same for t e same input of variable volta es, the method of controlling the output 0 unmodified-frequency components comprising varying the applied direct current voltages within said voltage operating range.

15. In a -modulator, a discharge device having a source of space current, a source of grid otential, and sources of waves for modu ation, said source of space current being of such voltage as to give the desired conductivity to said device, and said source of grid potential having a negative voltage approximating the smallest negative value for which the output of the modulation current component is dependent on said grid voltage.

16. In balanced thermionic relay systems in which a transmission characteristic of one relay for current'component of unmodified frequency is opposed to thatof another relay, means for compensating for differences in said characteristics comprising fixed operating voltagesfor said relays 'of such magnitude that the unmodified frequency output is made substantially a minimum independently of the output of the other components.

17. In balanced relay systems, comprising two thermionic relays 1n which for given input waves the output wave of each relay contains components of the same frequency asthe input waves and components of complex frequencies, and in which the relays duction of the components of complex frequencies in the desired amount is still obtainedJ: .1 3

In witness whereof, I hereunto subscribe my name this 27th day of September, A. 1)., 1919.

RALPH v. L. HARTLEY. 

